Particulate Cs-137 was collected from stream water for 2 years to assess the long-term trend of Cs-137 discharge from a forest after the Fukushima Nuclear Power Plant accident. A seasonal increase in the fluvial transport load of particulate Cs-137 in suspended solids (SS) was observed in July-October when rainfall was abundant. The Cs-137 load was controlled by the SS load. This control was attributed to cesium affinity for phyllosilicate clay minerals as verified by the low extractability of particulate Cs-137. These findings indicate the fluvial particulate Cs-137 load is significantly related to the climate and geomorphological features of Japan.